Fasteners: Fascinating

I had an interesting debate on the Amazon motorcycles mailing list the other day. It was about the first service of a motorcycle — break-in service.

Now, on the average bike (I’ll elaborate later) this consists of an oil change at around 600mi/1000km. The first bit on engine life all the internal parts are wearing in together. I would tend to argue that 600 miles is actually too long, but that’s a debate for another day. In any case you’re changing the oil to remove all the bits of the engine that have been machined away from rubbing against each other.

The spark for the debate was that the dealer would also check the frame bolts and stuff for tightness.

Why?

My contention is that if something was getting loose, it was either not engineered correctly or it wasn’t assembled correctly. I’ll take these in turn.

From an engineering perspective you know the stresses involved in the parts you are attaching. You should know the forces and stresses or else you should find a new line of work. Based on that you can determine how much clamping force is needed to overcome that. From there you choose the correct size and grade of fastener to meet that design goal.

The way bolts work is by either stretching the bolt itself or squishing the thing that is being bolted — actually both will occur. While typically we don’t think of metal as a “soft” thing, under high loads it’s quite malleable. The bolt will stretch a bit and the thing bolted will squish. If this happens a little (depending on grade of the metal) it’ll be an elastic deformation — if you undo it it’ll spring back to the original shape. If you do it a lot it’ll stretch to the point where it will go through plastic deformation and it won’t go back to it’s original shape. The stretch and squish are what fundamentally provides the force to keep the joint together.

If the joint was properly engineered it’ll hold.

Typically the clamping force provided by the nut will keep it on and overcome vibration and all that. If it doesn’t you need to go the extra step of providing another means for the nut to not back off. You can go with thread locker (Loctite for instance) that’ll give a higher breakaway torque than what was used to attach it. Alternately you can use one of a number of a mechanical means, like lock nuts or washers, deformable nuts, or distorted thread locknuts. Each of these will, if used correctly, prevent the fastener from backing out.

The other alternative (more likely I would imagine) is that it was just assembled incorrectly.

This is where we found some common ground: mistrust of dealers.

Techs typically work on a book rate. The book says something should take 45 minutes, you get paid for 45 minutes. Even if it takes you 10 minutes or 10 hours. It incents you to work fast. Sometimes fast is careless.

Now there are some fasteners that do need to get re-torqued. The head bolts on a BMW R (horizontally opposed twin) are long. A foot long give or take. As the engine breaks in things can shift a bit and it seats where it wants to. You need to loosen and re-tighten each head bolt to spec — to give it the right holding force for the rest of the engine’s life. From there you don’t need to touch them ever.

There’s also fasteners that are forced into plastic deformation. They’re single use things and a subject of another post. Those are cool too though.

This gets back to the original debate: you shouldn’t have to tighten bolts on a motorcycle because they shouldn’t just be getting loose. (Unless of course it’s documented) Do you regularly check the torque of all the fasteners you don’t see? How about on your car?

Either you trust the engineers and workers that build things, or you don’t.

Pick one.

If you don’t, don’t buy from them.

And in all cases, try to do your own work! There’s no one you can trust as much as yourself. You value your own life much more than anyone else does. Except maybe your mom — you can have her work on your ride I suppose.

3 Comments

Much weird reading I’ve done. I love this stuff for no really good reason… if I had a mech eng. degree I’d at least have an excuse (but probably no love). But I don’t. I’m just wired all wonky like that. Go figure.